This invention relates to a varistor comprising a sintered body containing a zinc oxide (ZnO) as a principal component. Particularly, it relates to a varistor having excellent life performance when a direct current is applied.
Various kinds of varistors (i.e., voltage-current non-linear resistors) for which extensive researches have been made include a varistor made of a sintered body containing ZnO as a principal component. In the case of such a varistor, it has been attempted to secure the desired performances by adding various kinds of auxiliary components.
Recently, researches and developments have been made on the direct current transmission, which, different from the alternating current transmission, impose very severe conditions upon the varistor because the electric field is applied to the non-linear resistor always in a single direction. Under the existing state of the art, however, there has been no varistor having a direct current life performance excellent enough to be useful under such severe conditions. For instance, known varistors are those comprising ZnO added with Bi.sub.2 O.sub.3, CoO, Sb.sub.2 O.sub.3, NiO and MnO as disclosed in Unexamined Patent Publication (KOKAI) No. 119188/1974, those comprising ZnO added with B and Bi as disclosed in Patent Publication (KOKOKU) No. 19472/1971 and those comprising ZnO added with a glass containing a boron oxide as disclosed in Patent Publication (KOKOKU) No. 33842/1981, etc., every of which, however, does not secure sufficient performance; for instance, they are poor in the life performance since the leakage current at the application of the direct current increases with lapse of time and the thermal runaway is caused thereby.
Moreover, demands for the performances such as the voltage-current non-linearity and the life performance have recently become severer as the ultra-high voltage (UHV) power transmission has made progress.
Thus, demands for improvements of the performances such as the life performance and the non-linearity have become larger year by year, and researches have been made everywhere in order to fulfill such demands.
This invention, made on account of the foregoing, has the object of providing a varistor having excellent direct current life performance. It is also an object of the present invention to provide a varistor having excellent voltage-current non-linearity. It is another object of the present invention to provide a varistor comprising a sintered body which contains a specific amount of .alpha.-phase Bi.sub.2 O.sub.3, but which is substantially free of chromium.